1873.] JXJDD THE SECONDAET BOCKS OF SCOTLAND. 167 



a particular stratum should be called a sandy shale or an argilla- 

 ceous sandstone. The thickness of beds passed through in the coal- 

 shafts at Brora was about 230 feet, and adding 70 feet more for the 

 clays seen in the river-cliff above the Eascally or Water- of- Brora 

 pit, we obtain for the total thickness of the argillaceous series above 

 the " Boof-bed " 300 feet. Some of the strata are well exposed in 

 a bluff of the river Brora, a little to the west of the Salmon-cruives, 

 where they are brought up in a double anticlinal fold (see fig. 4, p. 119), 

 aud the rocks can be examined by wading the river when it is low ; 

 other portions can be seen in a cliff to the eastward, and between 

 the last point and the Water-of- Brora pit ; and, lastly, the whole 

 series is displayed, though under somewhat unfavourable conditions, 

 in the reefs on the Inverbrora shore. The lower beds of this series 

 have been shown to belong to the zone of Ammonites calloviensis. 



In mineralogical characters the different portions of this great 

 series of clays offer very great variations. Sometimes we find dark 

 blue, highly pyritous, shaly clay, with septaria, the characters of 

 which recall the Oxford Clay of England ; in other places the beds 

 contain much wood in the form of jet, while nearly all the fossils 

 have disappeared through the decomposition of the pyrites by which 

 they were mineralized ; and throughout a considerable part of their 

 thickness the strata consist of finely laminated and very sandy shale 

 crowded with Belemnites, which often attain to a great size, and in- 

 numerable specimens of Ammonites, Gasteropoda, and Lamellibran- 

 chiata, lying compressed between the laminae. Occasionally the 

 beds last described pass into hard, fissile, bluish grey sandstone, 

 containing the same fossils, but in a worse state of preservation. 



About the geological horizon to which the greater part of this 

 series of argillaceous beds belongs, there is fortunately no reason for 

 entertaining doubt, the fossils, though often badly preserved, being 

 very numerous and highly characteristic. 



The Ammonites, which occur in great numbers, nearly all belong 

 to the group of the Ornatl ; but these are associated with a few 

 species of the groups of the Armati and Planulati. The Belemnites, 

 which are conspicuous alike from their vast numbers and the great 

 size which they attain, belong to the different varieties of Belemnites 

 Owenii of Pratt ; Belemnites sulcatus, Mill., also occurs, but is com- 

 paratively rare. Among the Gasteropoda we find Ceriihium mu- 

 7-icatum, Sow., and several species of Alaria ; while the Conchifera 

 are represented by many species, usually of small size, and often so 

 crushed as to defy specific determination. Oyster-banks do not ap- 

 pear to occur in this series of beds, though scattered specimens of 

 GnjpJicea dilatata, Sow. (small variety), are sometimes found. 

 Throughout the series wood, often in large masses, and converted 

 into jet, occurs in considerable abundance. 



The pakeontological characters of these beds clearly indicate that 

 they belong to a geological horizon which has been already studied 

 at many points in Germany, France, and England, and known as 

 the " Ornatenthon " or " brauncr Jura a " of Quenstedt, the 

 " zones of Ammonite anceps and aihleta " of Oppel and Hebert, and 



