352 Dr. C. Callawaij — On the Archcean Rocks. 



but sucli a conclusion would be premature. It is quite clear that 



the dark schist (e), which is found to the east of the fault, is the 



same band as that which underlies the granitoid rock (/) in the 



anticlinal. It therefore forms a link connecting the two areas, and 



enables us to piece together the rocks on each side of the fault into 



a continuous series, a, h, c, d, e, f. In other parts of Anglesey sections 



have been discovered which confirm this conclusion ; so that the 



gneissic rocks of this shattered district, instead of being regarded 



as a confused tangle of gnarled schist, are reduced to an orderly 



succession. The method, then, by which we may often obtain 



results in shattered districts is : 



(1). Ascertain clearly where the faults lie. 



(2). Find out the succession in the unfaulted areas. 



(3). Compare the unfaulted areas. If one or more rock-groups are 



common to two or more areas, a satisfactory result may often be 



obtained. 



The test by included fragments is of limited value. It simply 

 proves that the beds containing the fragments are newer than the 

 I'ock which supplied the fragments. In the younger formations this 

 test is altogether subordinate to the more decisive and precise testi- 

 mony of fossils, but where fossils are wanting it is often of great use. 

 If we found two formations in proximity, one a quartzite containing 

 Pentamerus ohlongus, and the other a shale with Ammonites bifrons, 

 we should at once assign the groups respectively to the May Hill 

 Sandstone and the Upper Lias. If the shale contained pebbles of 

 quartzite resembling the May Hill quartzite, we should hardly 

 trouble to notice the fact. The pebbles, if really derived from the 

 Silurian quartzite, would only prove that the shale was of any age 

 between the Silurian and the Recent. 



In studying the Archsean rocks, we often gain a great point if we 

 can prove that one group is newer than another. In Shropshire we 

 have two striking examples of the value of the test of included 

 fragments. Running through the heart of South Shropshire to the 

 south-west is a broken chain of wedge-like hills, of which the 

 Wrekin and Caer Caradoc are the most prominent. These ridges 

 are mainly built up of volcanic ashes and lavas. On both sides they 

 are bounded by faults ; numerous formations, from the Lower 

 Cambrian to the Lower Trias, being thrown against them. The 

 evidence from superposition is therefore indecisive. But in the 

 Longraynd (Lower Cambrian) hills, we find massive plum-coloui'ed 

 conglomerates made up of pebbles, some of tliem as large as a child's 

 head, most of which are a purple felstone identical in character with 

 one of the common types of the volcanic series. Plum-coloured sand- 

 stones overlie and underlie the conglomerates, and these rocks also 

 are made up chiefly of grains of the same felstone, with a small 

 proportion of quartz. It is thus evident that a considerable part of 

 the Lower Cambrian series in Shropshire has been derived from the 

 Wrekin volcanic group, which is in this way demonstz'ated to be 

 Pre-Cambrian or Archasan. 



The second example is of equal interest. At the south-western 



