186 MR. C. T. XRECHMANN OX A MASS OF ANHYDRITE [June 1 9 1 3- 



that material often of very large size, reaching several hundred- 

 weight, have frequently been observed in the Boulder Clay filling 

 the 'slack,' and are constantly brought up by the harbour-dredger 

 from the depression occupying the area between the two outcrops of 

 Magnesian Limestone, and also from the bed of the sea out towards 

 the bay. Their extension northwards and westwards is strictly- 

 limited to the area of the Hartlepool slack, but eastwards and 

 south-eastwards their distribution is uncertain ; though I have 

 been informed, on not very sure authority, that they extend across 

 the mouth of the Tees and have occurred as far south as Redcar. 



These masses of gypsum seem to be true glacial boulders, torn 

 up by a glacier passing in a south-easterly direction from the then 

 exposed surface of gypsum, and embedded in the Boulder Clay. 

 Before the coast was so largely encroached on by buildings, they 

 seem to have been exposed in several places, where the material was 

 largely quarried for carving small ornamental objects. 



The boulders are variable in colour, generally white but often 

 red, pink, or brown, and all seem to be pure gypsum without an- 

 hydrite or limestone. 



The promontory upon which Hartlepool is built is an isolated 

 outcrop of Upper Magnesian Limestone, and is roughly oval in its 

 present outline, lying with its longer axis oriented north-westwards 

 and south-eastwards, measuring about half a mile in width and 

 rather over a mile in length, from its first appearance at Parton 

 Rocks on the north-west to its disappearance beneath the sea on 

 the south-east near the end of the new breakwater. 



About five-sixths of a mile west of the exposure the Upper 

 Magnesian Limestone is again seen well exposed in the railway- 

 cutting near the gasworks at West Hartlepool ; and it is further 

 observable, skirting the harbour for a distance of nearly a mile. 



The intervening slack is filled in with Boulder Clay, upon which 

 rests a submerged forest or peat-bed, overlain in part by blown 

 sand and superficial deposits. In these Glacial and recent beds the 

 various harbours and timber-ponds of the two towns have been 

 excavated and constructed at different times. 



The first solid rock met with on piercing this covering of super- 

 ficial deposits is a bed of anhydrite of great thickness, which is 

 found to be altered to gypsum on both its upper and nether surfaces. 

 Unfortunately, but one boring is available showing the true 

 thickness of this bed, so our knowledge regarding its lateral 

 extension and thickness in other directions can be inferred only. 



There was nothing to indicate that the strata passed through 

 were in other than a horizontal position. If the uniform ' slack ' or 

 depression in which the harbours are constructed is entirely due, as 

 seems beyond question, to the erosion of the anhydrite, then the 

 mass should have an oblong or elliptical outline with its longer 

 axis lying north-west and south-east, and measuring possibly a mile 

 and a half in length and nearly a mile in width. 



The boring in question was sunk in 1888 by J. Vivian & Co., 

 of Whitehaven, by the diamond-boring process giving 6-inch cores,. 



