BASAL CAMBRIAN ROCKS OF SHROPSHIRE. 387 



Olenellusin the (Comley) Sandstone appears at first sight to fix dis- 

 (/inctly the Precambrian ago of the so-called Uriconian rocks of the 

 Wrekin and their English equivalents, and even to render the Pre- 

 cambrian age of the Longmynd a matter of fair probability." It 

 will be seen that in both these cases the age of the Longmynd 

 rocks was made to depend solely on their relations to the fossiliferous 

 sandstone; my suggestion, on the other hand, had arisen from en- 

 tirely different considerations. 



In any definite attempt to determine the age of a group of rocks 

 like those of the Longmynd, it appeared to me to be a necessary pre- 

 liminary to obtain a thorough knowledge of the rocks themselves ; 

 but I had not the slightest idea where their examination would lead 

 me. 



In their original description by Sir K. I. Murchison, in his ' Silurian 

 System,' they are treated as forming a continuous sequence, whose 

 base is to the east and whose summit is to the west, where they 

 are said to pass up into Lower-Silurian rocks. In this description the 

 lower part of the series is treated with care, but on the horizon of the 

 grits and conglomerates being reached, the remainder is lightly dis- 

 patched in the following words : — " These conglomerates and grits 

 form the central masses of the Longmynd, and are succeeded on the 

 west by alternations of similar strata. They are again followed 

 on the N.W. by various alternations of strata identical with those 

 described. Still further to the west, the purple-coloured grits and 

 sandstones and slaty schists prevail. The red grits are evidently of 

 regenerated origin, and often contain many small fragments of older 

 slate in a quartzose cement." 



It is the same with Mr. Salter in 1 857 *. He divides the beds 

 below the red grit into eight groups, but the entire remainder 

 is classed as No. 9. His eight subdivisions are not very easily made 

 to fit with the six of Sir P. I. Murchison ; but this is not to be 

 wondered at, both are quite true and are a matter of more or less 

 arbitrary grouping. He also shows in a section a perfect conformity 

 from the bottom to the top and a passage upwards into Lingula- 

 flags. The Survey section is practically identical with this. 



But geologists have been staggered at the enormous thickness 

 that such a section indicates — some 2(j,0()i) feet, the beds being, in 

 the lower part at least, highly inclined. And this result it has been 

 hoped to avoid by introducing one or more synclinals. But whether 

 such synclinals exist or not, no evidence has been adduced except by 

 Dr. Callaway. In a paper by that author entitled "The Precambrian 

 Bocks of Shropshire, part ii." J, a series of supposed Archa)an masses 

 along the western boundary of the Cambrian grits is described. 

 From their occurrence it is presumed that we are here at the base 

 of the Cambrian instead of its summit, and it is stated that on the 

 east side of the line the Longmynd rocks dip easterly, of which one 

 example (but one only) is given near Lyd's Hole. 



As far as I can gather, the memoirs and maps above referred to 



* Quart. Journ. Geo), fcjoc. vol. xiii. 

 t Ibid. vol. xxxviii. 



