HORIZONTAL SEQUENCE OF STRATA. 51 



Other minerals which occur abundantly in some crystalline rocks, 

 such as hornblende and augite, yield iron ore, and whenever volcanic 

 rocks, especially basalts, decay, a deposit of iron ore is formed from 

 the iron they contained. Professor Kamsay has, in North Wales, 

 shown that the volcanic rocks associated with the Tremadoc beds, 

 on being traced to some distance, pass into a deposit of pisolitic iron 

 ore ; and a relation has been observed between the iron ores and the 

 basalts of County Antrim in Ireland. These groups of minerals, 

 micas, hornblendes, and augites, also contain a large amount of 

 magnesia, and when a rock like the magnesian limestone or 

 dolomite of the Permian series is met with, it is reasonable to 

 suppose that the magnesia was derived originally from the decay 

 of minerals which contained that substance. The crystalline rocks 

 are also the only known source of lime, and it has already been 

 indicated that the decaying vegetation on the sea-shore charges the 

 water with a sufficient amount of carbonic acid to enable it to 

 dissolve the lime and hold it suspended in an invisible form. 

 Where the rivers bring down much lime, or where the coasts are 

 very slowly worn away, or where animal life abounds exuberantly 

 near to the coast, deposits of limestone accumulate close to shore. 

 The Coralline Crag in the east of Suffolk is an organic deposit of 

 this kind, and deposits similar to that one are said to be forming 

 near to Tierra del Fuego. But when the shore limestone results 

 chiefly from evaporation of the sea, the deposit is necessarily largely 

 composed of materials which have no relation to living structures. 

 And it follows from what has already been said about the dis- 

 tribution of sands and clays, that when we go out to sea beyond 

 the limits to which sediments are carried from the shore, the only 

 deposit forming on the bot'tom will be limestones, chiefly constructed 

 from the skeletons of animals which live in the ocean. 



Horizontal Sequence of Rocks. We have here been considering 

 in a general way the order in which deposits become arranged on the 

 sea-bed when the parent rock is granite or some such crystalline sub- 

 stance. Bearing this in mind, it will be evident that as the shore is 

 receded from, there is a necessary horizontal sequence ofrocJcs in the order: 

 sand, clay, limestone. Of course, it is possible that the coast might 

 consist entirely of quartzite, which is a rock formed of quartz, when 

 no clay could possibly be produced by wearing it away ; or the shore 

 rocks might be formed of lavas in which little or no quartz exists in 

 grains, and then of course no sand can be produced, and the whole rock 

 will break up into clay. And irregularities of the sea-bed and animal 

 growth may cause limestones to accumulate which are quite indepen- 

 dent of the positions of clays and sands. Such deposits, however, are 

 rather the exception than the rule. And it may be held as generally 

 true that all crystalline rocks decay into materials which become sand- 

 stones, clays, and limestones. And when a cliff composed of layers of 

 these rocks, like . the Yorkshire coast, comes again to be worn away 

 and separated into the mineral substances of which it consists, the 

 materials will be sorted out as before and rearranged parallel to the 

 coast in the order sand, clay, limestone. There are many examples 



