Due to early evaporation of moisture in the clouds

when cloud has water in it so it appears black and when it 
has not water so it appears as white

A rock is any large massif the harder potion of the earth?s
crust- in word we ca say that one if the skid materials . in
the some case it consists of only a single mineral but
generally of several minerals. A  rock however dies not
normally have a definite chemical composition as a mineral does

                         The rocks may be classified into
four main types according to the way in which they have been
forms as follows.

                      (I)Stratifies or Sedimentary Rocks:-
There are the  rocks which have been laid down in beds or
layers(strata) by wind, running water or the seas..  These
rocks are important in many ways as  75% part of the earth
is made up of these rocks

                       Formation :-Plants and animals might
sometimes have been buried and their hard part might left
therein. Lather in the sediments were raised by earth-quakes
into dry land. These ?remains?  one  knows as ?fossils?. We
can side this type of rocks into 3 groups.

i.	 Alluvium which are still firmed by rivers
ii.	Young soft rocks   which are not subject to great
earth-quakes or mountaian-bulding movement oil is found in
such rocks
iii.	Old hard rocks  which have not been folded bent and
cracked by many earth-quakes occurred and they often raised
into high mountains. Amongst the soft rocks, we usually find
hard bands which gave rise to water-falls etc

                     (2)Igneous Rocks  this type of  rocks 
are also known as fiery rocks are formed due to interior
heat if the earth. More rocks are if two types

i.	Volcanic rocks  These are the rocks which reach the
surface in   Volcanoes. In other countries the greate areas
if rock in the Deccan around Bombay have originated in this way
ii.	Thore rocks . There are the rocks which moved
under-ground but became solid before they reached the
surface A good example of these rocks is Granite. Such rocks
are generally hard and resist weathering.

(3)Meteamorphic Rocks  This means the rocks which have
changed their forms. We know that earth-quake bend and fold
the earth?s crust and gradually build up great mountains. In
the process the rocks ate changed and hardened. Other rocks
become buried by the fresh layers piled in the top of heat
in the lower part of the earth?s crust. We  all know that if
we take a piece of clay and bake if , the  brick which
results is quite different from the clay. This is a the
rocks might have altered far more. Therefore, it is not
possible to say what the shape and composition of original
rock. We call such rocks as metamorphic rocks. It takes a
pretty long time to get changed. Some if the areas of
crystalline rocks were firmed ages and ages ago and have
remained unchanged ever since

         In our country, the Deccan in a nice example of
this type of rocks.
(4) Organic Rocks These rocks are formed of the remains by
or of, animals or plants(organisms). Coal is an organic rock
which is formed from the remains of forests which have been
formerly submerged and later on buried. Mineral oils are
also formed its load of mud in the sea or a lake.