OF CREATION. 23 



CHAPTER III. 



THE PERIOD'OF THE EXISTENCE OF INVERTEBRATED ANIMALS AS THE 

 MOST HIGHLY ORGANIZED INHABITANTS OF THE SEA. THE SILU- 

 RIAN ROCKS. 



WRAPPING round the igneous rocks of Cumberland 

 and the lake district, ranging over a considerable part 

 of the north-east of Ireland, occupying a large portion 

 of South Wales, and present almost everywhere in 

 North Wales, there are found a great number of sedi- 

 mentary rocks of various kinds, covering the gneiss, 

 mica schist, or clay slate, and covered up in South 

 Wales by a series of coarse red conglomerates or beds 

 of pudding-stone. These sedimentary rocks are ex- 

 panded sometimes to a thickness of many thousand 

 feet, and they form a remarkable and natural group, 

 which may be conveniently sub-divided into two 

 parts, the lower being by far the most considerable 

 in vertical thickness, but the upper containing a 

 greater number and variety of the fossil remains of 

 animals. 



In the British Islands, and very generally in other 

 countries, this lower group of rocks consists of a gray- 

 ish-coloured sandy stone, often slaty or flaggy, and 

 containing much clayey matter, sometimes including 

 poor bands of limestone, and not unfrequently exhi- 

 biting, in the partings between two beds, a number of 

 imperfect remains of shells and other organic sub- 



