DESCRIPTION OF THE FORMATIONS 373 



Tracing these beds toward the north they are found to lose the coarse pebbly 

 and conglomeratic features and to pass into fine flaggy sandstones, with sandy 

 shales, the whole having a prevailing dull chocolate-red tint, through which 

 seams of greenish-gray shales and flagstones occur. In places, as observed by 

 Sedgwick and Murchison, the red tint is local and even superficial, the same 

 stratum being red or green at different parts of its course. Traced inland to 

 the west the beds so increase in the quantity of coarse detritus that on the 

 north side of the Scarabin ridge hundreds of feet may either be called highly 

 conglomeratic sandstones or sandy conglomerates. 



"The four groups of strata, above described, may be regarded as forming 

 together a red sandy and conglomeratic base, of very variable thickness, on 

 which lies the great flagstone series of Caithness now to be discussed" (page 

 386). 



5. Wick, or Eastern Flagstone Group 5,000 feet 



This group consists of dark-gray flagstones which are often thick-bedded, 

 thin shales and limestone bands ; the whole passing down into red shales and 

 sandstones. 



The group is distinguished from that which overlies it by the greater mas- 

 siveness of its flagstones, and by their less calcareous composition and less 

 fissile or shaly texture. 



Sun-cracks and ripple-marks abound and in various horizons remains of 

 terrestrial plants have been found, one gray shale in particular having its 

 surface covered with carbonized vegetation. Large stems belonging to tree 

 ferns and gymnosperms also occur. An abundant fish fauna has been obtained 

 from this formation. 



6. Thurso, or Northern Flagstone Group 5,000 feet 



Dark-gray and cream-colored flagstones, gray and blue shales and thin lime- 

 stone ; some beds strongly bituminous. This group is more fissile, shaly, and 

 calcareous than the preceding. "The flagstones which, towards the east, retain 

 the usual normal characters of fissile calcareous strata, pass into sandstones 

 and conglomerates as they approach and rest upon the granite and gneiss" 

 (page 391) . 



"The nest feature to engage the attention of the observer is probably the 

 extraordinary abundance of ripple-marked surfaces and sun-cracks. Though 

 these markings abound also in the lower flagstone group, it is here that they 

 attain their greatest development" (page 392). 



"More abundant and admirable illustrations of sun-cracks could hardly be 

 found than occur along this coast. Broad gently-inclined sheets of rock again 

 and again present themselves to view so covered with reticulations as to look 

 like tesselated pavements. It may be noticed that the cracks not infrequently 

 descend through many of the fine laminae of deposit for a depth of five or six 

 inches with occasionally a breadth of three or four inches. The material filling 

 up the interstices abounds with small, occasionally curved pieces of shale. 

 These may, no doubt, be regarded as portions of the upper muddy layer which 

 cracked off and curled up during desiccation, as may often be observed on 

 dried-up pools at the present time. Some pittings, occasionally seen on the 

 sun-cracked surfaces, may perhaps represent rain-drops. Altogether, no evi- 

 dence could more conclusively indicate a long-continued, tranquil deposit of 



